She is generous and kind but no pushover, and her steely determination to do the right thing by those she loves has earned her respect from everyone at Downton.

Even hard-hearted O’Brien grudgingly accepts that Anna is good at her job and does not cross her. Only Thomas will occasionally send a barbed remark in her direction, but it’s usually about Bates.

‘Anna is emotionally wise for her age,’ says Joanne Froggatt, the actress who plays her.

‘I think she must have gone through things in her childhood that have taught her to deal with hard times in the way that she does.’ She has a backbone of steel and is determined to uncover something the detective on her husband’s case missed.

In her quest to unlock the mystery, she might have drawn comfort from The Mysterious Affair at Styles, Agatha Christie’s first novel, which was published that year and introduced the detective Hercule Poirot.

It suggested that things are not as they first appear, and that intelligence and persistence are required to uncover the true course of events.

As head housemaid, Anna is responsible for certain lady's maid duties for the daughters of the house, as well as for any female guest who arrives to stay without her own personal servant

But while Anna is relentless in her sleuthing, she cannot do it all the time, and she may be grateful for the distraction her work brings.

With a respectable position but less seniority than Mrs Hughes, Anna is able to move easily between the floors of the house, more so than anyone else.

She has genuine relationships across the spectrum – from the junior maids below stairs to her attachment to Lady Mary above. Molesley, the butler/valet for Mrs Crawley and Matthew doesn’t have much time for most people but he has a soft spot for Anna.

Even Lord Grantham is fond of her, as the loved one of his long-suffering valet and friend. He feels deeply sorry for her, too – despite his best efforts, Bates is sitting out a sentence that could last up to 20 years.

As head housemaid, Anna is responsible for certain lady’s maid duties for the daughters of the house, as well as for any female guest who arrives to stay without her own personal servant.

The head housemaid would look after the married women; the junior maids would take care of the unmarried guests.

Now that Lady Mary is planning to get married, she wants Anna formally appointed as her own lady’s maid. One wonders whether Mary’s feelings for her are reciprocated.

‘She has a close relationship with Lady Mary,’ says Froggatt. ‘But she understands that her world is not just about material things. Everyone’s life has its problems, she prefers the world she knows.’

A replacement head housemaid cannot be found quickly enough to release her. So for now, Anna must juggle her duties, which is not an easy task. When cleaning the rooms in the morning, before the family had got up, the head housemaid was expected to dust the most valuable objects and the finest pieces of furniture.

For a woman servant, marriage often meant the end of her life in service – but it was only with the arrival of children that this became inevitable

There will be many new responsibilities to take on when she becomes a lady’s maid proper. If Anna were to join Lady Mary on trips she would be expected to oversee many of the practicalities of the journey, starting with the packing.

As one lady’s maid recalled, this was a process fraught with pitfalls, ‘Choosing what to take wasn’t easy. Before leaving, mistresses could be a bit short, “Oh, the usual things, you know what I like”, but when you get to the other end and you haven’t brought what they want, it’s a very different story.’

With her husband in prison, the question of being married while in service is not raised for now – and Lord Grantham has promised them a cottage on the estate.

For a woman servant, marriage often meant the end of her life in service – but it was only with the arrival of children that this became inevitable.

With Bates behind bars, this doesn’t look likely for Anna for a while. Until Bates is released, her own future is as bleak as that of her husband. Despite the many obstructions they have already overcome, this is the hardest challenge of all. But if anyone can do it, Anna can.